[PATCH next 2/2] spi: spi-qpic-snand: add support for 8 bits ECC strength

[Gabor Juhos]

2025. 05. 12. 10:32 keltezéssel, Miquel Raynal írta:
> On 05/05/2025 at 15:21:52 +02, Gabor Juhos <j4g8y7 at gmail.com> wrote:
> 
>> 2025. 05. 05. 13:17 keltezéssel, Md Sadre Alam írta:
>>>
>>>
>>

[...]

>> [   48.940586] ==================================================
>> [   48.941112] mtd_nandbiterrs: MTD device: 0
>> [   48.946811] mtd_nandbiterrs: MTD device size 268435456, eraseblock=131072,
>> page=2048, oob=128
>> [   48.950837] mtd_nandbiterrs: Device uses 1 subpages of 2048 bytes
>> [   48.959448] mtd_nandbiterrs: Using page=0, offset=0, eraseblock=0
>> [   48.969014] mtd_nandbiterrs: incremental biterrors test
>> [   48.971596] mtd_nandbiterrs: write_page
>> [   48.977594] mtd_nandbiterrs: rewrite page
>> [   48.981277] mtd_nandbiterrs: read_page
>> [   48.985247] mtd_nandbiterrs: verify_page
>> [   48.988269] mtd_nandbiterrs: Successfully corrected 0 bit errors per subpage
>> [   48.992327] mtd_nandbiterrs: Inserted biterror @ 0/5
>> [   48.999428] mtd_nandbiterrs: rewrite page
>> [   49.005508] mtd_nandbiterrs: read_page
>> [   49.008836] mtd_nandbiterrs: Read reported 1 corrected bit errors
>> [   49.011858] mtd_nandbiterrs: verify_page
>> [   49.018061] mtd_nandbiterrs: Successfully corrected 1 bit errors per subpage
>> [   49.022015] mtd_nandbiterrs: Inserted biterror @ 0/2
>> [   49.029058] mtd_nandbiterrs: rewrite page
>> [   49.034848] mtd_nandbiterrs: read_page
>> [   49.038516] mtd_nandbiterrs: Read reported 1 corrected bit errors
>> [   49.041545] mtd_nandbiterrs: verify_page
>> [   49.047755] mtd_nandbiterrs: Successfully corrected 2 bit errors per subpage
>> [   49.051702] mtd_nandbiterrs: Inserted biterror @ 0/0
>> [   49.058760] mtd_nandbiterrs: rewrite page
>> [   49.064565] mtd_nandbiterrs: read_page
>> [   49.068203] mtd_nandbiterrs: Read reported 1 corrected bit errors
>> [   49.071232] mtd_nandbiterrs: verify_page
>> [   49.077432] mtd_nandbiterrs: Successfully corrected 3 bit errors per subpage
>> [   49.081389] mtd_nandbiterrs: Inserted biterror @ 1/7
>> [   49.088432] mtd_nandbiterrs: rewrite page
>> [   49.094197] mtd_nandbiterrs: read_page
>> [   49.097889] mtd_nandbiterrs: Read reported 2 corrected bit errors
>> [   49.100919] mtd_nandbiterrs: verify_page
>> [   49.107121] mtd_nandbiterrs: Successfully corrected 4 bit errors per subpage
>> [   49.111077] mtd_nandbiterrs: Inserted biterror @ 1/5
>> [   49.118137] mtd_nandbiterrs: rewrite page
>> [   49.123892] mtd_nandbiterrs: read_page
>> [   49.127576] mtd_nandbiterrs: Read reported 2 corrected bit errors
>> [   49.130608] mtd_nandbiterrs: verify_page
>> [   49.136808] mtd_nandbiterrs: Successfully corrected 5 bit errors per subpage
>> [   49.140764] mtd_nandbiterrs: Inserted biterror @ 1/2
>> [   49.147807] mtd_nandbiterrs: rewrite page
>> [   49.153607] mtd_nandbiterrs: read_page
>> [   49.157268] mtd_nandbiterrs: Read reported 2 corrected bit errors
>> [   49.160294] mtd_nandbiterrs: verify_page
>> [   49.166496] mtd_nandbiterrs: Successfully corrected 6 bit errors per subpage
>> [   49.170452] mtd_nandbiterrs: Inserted biterror @ 1/0
>> [   49.177498] mtd_nandbiterrs: rewrite page
>> [   49.183253] mtd_nandbiterrs: read_page
>> [   49.186974] mtd_nandbiterrs: Read reported 2 corrected bit errors
>> [   49.189983] mtd_nandbiterrs: verify_page
>> [   49.196194] mtd_nandbiterrs: Successfully corrected 7 bit errors per subpage
>> [   49.200140] mtd_nandbiterrs: Inserted biterror @ 2/6
>> [   49.207182] mtd_nandbiterrs: rewrite page
>> [   49.212895] mtd_nandbiterrs: read_page
>> [   49.216671] mtd_nandbiterrs: Read reported 3 corrected bit errors
>> [   49.219670] mtd_nandbiterrs: verify_page
>> [   49.225872] mtd_nandbiterrs: Successfully corrected 8 bit errors per subpage
>> [   49.229827] mtd_nandbiterrs: Inserted biterror @ 2/5
>> [   49.236871] mtd_nandbiterrs: rewrite page
>> [   49.242629] mtd_nandbiterrs: read_page
>> [   49.246348] mtd_nandbiterrs: error: read failed at 0x0
>> [   49.249356] mtd_nandbiterrs: After 9 biterrors per subpage, read reported
>> error -74
>> [   49.257958] mtd_nandbiterrs: finished successfully.
>> [   49.262029] ==================================================
>> #
> 
> This is not the intended behavior. The reporting is wrong and should
> report an increasing number of bitflips instead of 1, 1, 1, 2, 2, 2, 2,
> 3.
> 
> With this behavior, wear levelling will not work correctly.
> 
> This is not related to Gabor's changes, but I believe this should be
> updated otherwise it does not make much sense to increase the strength.

It still makes some sense. For example, the bootloader on my board uses 8 bits
strength when it handles the NAND flash. Without supporting that, the kernel
can't read the data written by the bootloader and vice versa.

> Gabor, can you look into it? Both patches look fine otherwise, thanks a
> lot!

I have checked it. According to the code, the driver reads the reported values
from a hardware register.

Here is the result of another test but i have added some debug code to the
driver so the output contains the hardware register values:

  # insmod mtd_nandbiterrs dev=0
  [   64.791395] 
  [   64.791423] ==================================================
  [   64.791950] mtd_nandbiterrs: MTD device: 0
  [   64.797592] mtd_nandbiterrs: MTD device size 268435456, eraseblock=131072, page=2048, oob=128
  [   64.801779] mtd_nandbiterrs: Device uses 1 subpages of 2048 bytes
  [   64.810313] mtd_nandbiterrs: Using page=0, offset=0, eraseblock=0
  [   64.820832] mtd_nandbiterrs: incremental biterrors test
  [   64.822452] mtd_nandbiterrs: write_page
  [   64.828511] mtd_nandbiterrs: rewrite page
  [   64.832270] mtd_nandbiterrs: read_page
  [   64.836184] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0200 erased_cw:00000002
  [   64.839091] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   64.847364] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   64.855694] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   64.864020] qcom_snand 79b0000.spi: corrected:0 failed:0 bitflips:0
  [   64.872349] mtd_nandbiterrs: verify_page
  [   64.878598] mtd_nandbiterrs: Successfully corrected 0 bit errors per subpage
  [   64.882760] mtd_nandbiterrs: Inserted biterror @ 0/5
  [   64.889793] mtd_nandbiterrs: rewrite page
  [   64.895541] mtd_nandbiterrs: read_page
  [   64.899228] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0201 erased_cw:00000002
  [   64.902282] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   64.910549] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   64.918860] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   64.927206] qcom_snand 79b0000.spi: corrected:1 failed:0 bitflips:1
  [   64.935552] mtd_nandbiterrs: Read reported 1 corrected bit errors
  [   64.941789] mtd_nandbiterrs: verify_page
  [   64.948043] mtd_nandbiterrs: Successfully corrected 1 bit errors per subpage
  [   64.952042] mtd_nandbiterrs: Inserted biterror @ 0/2
  [   64.959048] mtd_nandbiterrs: rewrite page
  [   64.964815] mtd_nandbiterrs: read_page
  [   64.968490] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0201 erased_cw:00000002
  [   64.971586] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   64.979814] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   64.988131] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   64.996481] qcom_snand 79b0000.spi: corrected:1 failed:0 bitflips:1
  [   65.004810] mtd_nandbiterrs: Read reported 1 corrected bit errors
  [   65.011059] mtd_nandbiterrs: verify_page
  [   65.017313] mtd_nandbiterrs: Successfully corrected 2 bit errors per subpage
  [   65.021307] mtd_nandbiterrs: Inserted biterror @ 0/0
  [   65.028319] mtd_nandbiterrs: rewrite page
  [   65.034131] mtd_nandbiterrs: read_page
  [   65.037862] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0201 erased_cw:00000002
  [   65.040844] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.049069] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.057418] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.065763] qcom_snand 79b0000.spi: corrected:1 failed:0 bitflips:1
  [   65.074082] mtd_nandbiterrs: Read reported 1 corrected bit errors
  [   65.080341] mtd_nandbiterrs: verify_page
  [   65.086584] mtd_nandbiterrs: Successfully corrected 3 bit errors per subpage
  [   65.090574] mtd_nandbiterrs: Inserted biterror @ 1/7
  [   65.097589] mtd_nandbiterrs: rewrite page
  [   65.103374] mtd_nandbiterrs: read_page
  [   65.107030] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0202 erased_cw:00000002
  [   65.110120] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.118340] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.126689] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.135032] qcom_snand 79b0000.spi: corrected:2 failed:0 bitflips:2
  [   65.143357] mtd_nandbiterrs: Read reported 2 corrected bit errors
  [   65.149611] mtd_nandbiterrs: verify_page
  [   65.155855] mtd_nandbiterrs: Successfully corrected 4 bit errors per subpage
  [   65.159844] mtd_nandbiterrs: Inserted biterror @ 1/5
  [   65.166860] mtd_nandbiterrs: rewrite page
  [   65.172645] mtd_nandbiterrs: read_page
  [   65.176301] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0202 erased_cw:00000002
  [   65.179389] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.187611] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.195961] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.204289] qcom_snand 79b0000.spi: corrected:2 failed:0 bitflips:2
  [   65.212623] mtd_nandbiterrs: Read reported 2 corrected bit errors
  [   65.218858] mtd_nandbiterrs: verify_page
  [   65.225139] mtd_nandbiterrs: Successfully corrected 5 bit errors per subpage
  [   65.229102] mtd_nandbiterrs: Inserted biterror @ 1/2
  [   65.236147] mtd_nandbiterrs: rewrite page
  [   65.241899] mtd_nandbiterrs: read_page
  [   65.245572] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0202 erased_cw:00000002
  [   65.248635] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.256902] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.265234] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.273561] qcom_snand 79b0000.spi: corrected:2 failed:0 bitflips:2
  [   65.281895] mtd_nandbiterrs: Read reported 2 corrected bit errors
  [   65.288129] mtd_nandbiterrs: verify_page
  [   65.294408] mtd_nandbiterrs: Successfully corrected 6 bit errors per subpage
  [   65.298373] mtd_nandbiterrs: Inserted biterror @ 1/0
  [   65.305413] mtd_nandbiterrs: rewrite page
  [   65.311170] mtd_nandbiterrs: read_page
  [   65.314843] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0202 erased_cw:00000002
  [   65.317906] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.326170] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.334501] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.342833] qcom_snand 79b0000.spi: corrected:2 failed:0 bitflips:2
  [   65.351164] mtd_nandbiterrs: Read reported 2 corrected bit errors
  [   65.357399] mtd_nandbiterrs: verify_page
  [   65.363680] mtd_nandbiterrs: Successfully corrected 7 bit errors per subpage
  [   65.367643] mtd_nandbiterrs: Inserted biterror @ 2/6
  [   65.374684] mtd_nandbiterrs: rewrite page
  [   65.380445] mtd_nandbiterrs: read_page
  [   65.384112] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0203 erased_cw:00000002
  [   65.387177] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.395445] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.403769] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.412103] qcom_snand 79b0000.spi: corrected:3 failed:0 bitflips:3
  [   65.420436] mtd_nandbiterrs: Read reported 3 corrected bit errors
  [   65.426671] mtd_nandbiterrs: verify_page
  [   65.432950] mtd_nandbiterrs: Successfully corrected 8 bit errors per subpage
  [   65.436915] mtd_nandbiterrs: Inserted biterror @ 2/5
  [   65.443958] mtd_nandbiterrs: rewrite page
  [   65.449719] mtd_nandbiterrs: read_page
  [   65.453386] qcom_snand 79b0000.spi: cw[0] flash:00003030 buffer:00ff0301 erased_cw:00000002
  [   65.456448] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   65.464715] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   65.473041] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   65.481374] qcom_snand 79b0000.spi: corrected:0 failed:1 bitflips:3
  [   65.489706] mtd_nandbiterrs: error: read failed at 0x0
  [   65.495941] mtd_nandbiterrs: After 9 biterrors per subpage, read reported error -74
  [   65.504609] mtd_nandbiterrs: finished successfully.
  [   65.508700] ==================================================


Basically, the driver reads the page as 4 codewords along with reading the
register values after each. The reported values are coming from the low 
5 bits of the 'buffer' values printed above. The layout of the register is
barely documented in the driver, so the exact meaning of the bits is unknown.

Despite that, I had an idea, so I have changed the code in the nandbiterrs
module to insert only single bit error into a given byte. By doing the test
with the modified module results in the following:

  # insmod mtd_nandbiterrs dev=0
  [   37.125416] 
  [   37.125444] ==================================================
  [   37.125970] mtd_nandbiterrs: MTD device: 0
  [   37.131764] mtd_nandbiterrs: MTD device size 268435456, eraseblock=131072, page=2048, oob=128
  [   37.135695] mtd_nandbiterrs: Device uses 1 subpages of 2048 bytes
  [   37.144314] mtd_nandbiterrs: Using page=0, offset=0, eraseblock=0
  [   37.155047] mtd_nandbiterrs: incremental biterrors test
  [   37.156481] mtd_nandbiterrs: write_page
  [   37.162518] mtd_nandbiterrs: rewrite page
  [   37.166298] mtd_nandbiterrs: read_page
  [   37.170193] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0200 erased_cw:00000002
  [   37.173111] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.181383] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.189708] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.198020] qcom_snand 79b0000.spi: corrected:0 failed:0 bitflips:0
  [   37.206367] mtd_nandbiterrs: verify_page
  [   37.212631] mtd_nandbiterrs: Successfully corrected 0 bit errors per subpage
  [   37.216768] mtd_nandbiterrs: Inserted biterror @ 1/7
  [   37.223809] mtd_nandbiterrs: rewrite page
  [   37.229569] mtd_nandbiterrs: read_page
  [   37.233244] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0201 erased_cw:00000002
  [   37.236301] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.244571] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.252896] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.261227] qcom_snand 79b0000.spi: corrected:1 failed:0 bitflips:1
  [   37.269560] mtd_nandbiterrs: Read reported 1 corrected bit errors
  [   37.275795] mtd_nandbiterrs: verify_page
  [   37.282075] mtd_nandbiterrs: Successfully corrected 1 bit errors per subpage
  [   37.286039] mtd_nandbiterrs: Inserted biterror @ 3/7
  [   37.293080] mtd_nandbiterrs: rewrite page
  [   37.298877] mtd_nandbiterrs: read_page
  [   37.302531] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0202 erased_cw:00000002
  [   37.305572] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.313839] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.322169] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.330498] qcom_snand 79b0000.spi: corrected:2 failed:0 bitflips:2
  [   37.338818] mtd_nandbiterrs: Read reported 2 corrected bit errors
  [   37.345078] mtd_nandbiterrs: verify_page
  [   37.351352] mtd_nandbiterrs: Successfully corrected 2 bit errors per subpage
  [   37.355310] mtd_nandbiterrs: Inserted biterror @ 5/7
  [   37.362352] mtd_nandbiterrs: rewrite page
  [   37.368094] mtd_nandbiterrs: read_page
  [   37.371814] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0203 erased_cw:00000002
  [   37.374843] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.383111] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.391437] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.399769] qcom_snand 79b0000.spi: corrected:3 failed:0 bitflips:3
  [   37.408089] mtd_nandbiterrs: Read reported 3 corrected bit errors
  [   37.414351] mtd_nandbiterrs: verify_page
  [   37.420616] mtd_nandbiterrs: Successfully corrected 3 bit errors per subpage
  [   37.424581] mtd_nandbiterrs: Inserted biterror @ 7/7
  [   37.431622] mtd_nandbiterrs: rewrite page
  [   37.437419] mtd_nandbiterrs: read_page
  [   37.441072] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0204 erased_cw:00000002
  [   37.444114] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.452380] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.460708] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.469047] qcom_snand 79b0000.spi: corrected:4 failed:0 bitflips:4
  [   37.477360] mtd_nandbiterrs: Read reported 4 corrected bit errors
  [   37.483621] mtd_nandbiterrs: verify_page
  [   37.489888] mtd_nandbiterrs: Successfully corrected 4 bit errors per subpage
  [   37.493852] mtd_nandbiterrs: Inserted biterror @ 8/7
  [   37.500893] mtd_nandbiterrs: rewrite page
  [   37.506636] mtd_nandbiterrs: read_page
  [   37.510342] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0205 erased_cw:00000002
  [   37.513385] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.521652] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.529978] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.538297] qcom_snand 79b0000.spi: corrected:5 failed:0 bitflips:5
  [   37.546643] mtd_nandbiterrs: Read reported 5 corrected bit errors
  [   37.552896] mtd_nandbiterrs: verify_page
  [   37.559161] mtd_nandbiterrs: Successfully corrected 5 bit errors per subpage
  [   37.563123] mtd_nandbiterrs: Inserted biterror @ 10/7
  [   37.570168] mtd_nandbiterrs: rewrite page
  [   37.575956] mtd_nandbiterrs: read_page
  [   37.579702] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0206 erased_cw:00000002
  [   37.582743] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.591010] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.599354] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.607655] qcom_snand 79b0000.spi: corrected:6 failed:0 bitflips:6
  [   37.616004] mtd_nandbiterrs: Read reported 6 corrected bit errors
  [   37.622249] mtd_nandbiterrs: verify_page
  [   37.628504] mtd_nandbiterrs: Successfully corrected 6 bit errors per subpage
  [   37.632497] mtd_nandbiterrs: Inserted biterror @ 12/7
  [   37.639524] mtd_nandbiterrs: rewrite page
  [   37.645353] mtd_nandbiterrs: read_page
  [   37.649047] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0207 erased_cw:00000002
  [   37.652100] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.660367] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.668679] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.677029] qcom_snand 79b0000.spi: corrected:7 failed:0 bitflips:7
  [   37.685358] mtd_nandbiterrs: Read reported 7 corrected bit errors
  [   37.691607] mtd_nandbiterrs: verify_page
  [   37.697861] mtd_nandbiterrs: Successfully corrected 7 bit errors per subpage
  [   37.701852] mtd_nandbiterrs: Inserted biterror @ 14/7
  [   37.708867] mtd_nandbiterrs: rewrite page
  [   37.714683] mtd_nandbiterrs: read_page
  [   37.718397] qcom_snand 79b0000.spi: cw[0] flash:00003020 buffer:00ff0208 erased_cw:00000002
  [   37.721478] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.729718] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.738037] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.746433] qcom_snand 79b0000.spi: corrected:8 failed:0 bitflips:8
  [   37.754719] mtd_nandbiterrs: Read reported 8 corrected bit errors
  [   37.760972] mtd_nandbiterrs: verify_page
  [   37.767218] mtd_nandbiterrs: Successfully corrected 8 bit errors per subpage
  [   37.771214] mtd_nandbiterrs: Inserted biterror @ 17/7
  [   37.778225] mtd_nandbiterrs: rewrite page
  [   37.784054] mtd_nandbiterrs: read_page
  [   37.787754] qcom_snand 79b0000.spi: cw[0] flash:00003030 buffer:00ff0301 erased_cw:00000002
  [   37.790836] qcom_snand 79b0000.spi: cw[1] flash:00002020 buffer:00ff0200 erased_cw:00000002
  [   37.799076] qcom_snand 79b0000.spi: cw[2] flash:00001020 buffer:00ff0200 erased_cw:00000002
  [   37.807395] qcom_snand 79b0000.spi: cw[3] flash:00000020 buffer:00ff0200 erased_cw:00000002
  [   37.815743] qcom_snand 79b0000.spi: corrected:0 failed:1 bitflips:8
  [   37.824073] mtd_nandbiterrs: error: read failed at 0x0
  [   37.830321] mtd_nandbiterrs: After 9 biterrors per subpage, read reported error -74
  [   37.838993] mtd_nandbiterrs: finished successfully.
  [   37.843068] ==================================================

Interestingly enough, it reports the correct number of bit errors now.
For me it seems, that the hardware reports the number of the corrected
*bytes* instead of the corrected *bits*.

I can't figure out whether this is a configuration problem only, or it
is a hardware limitation.

Nevertheless, it is possible that I'm entirely wrong about this, since
I have no documentation for the hardware.

Maybe Alam has some ideas about what could be wrong.

Regards,
Gabor